The present invention relates to foamed plastic products and in particular to foamed plastic products having an outer protective skin which is extruded onto a resilient foamed product. The present invention is also directed to a method of manufacturing of the product.
Various types of foamed plastic products are known and many of these products are produced by an extrusion process and produce a body portion which is relatively soft and resilient. Foamed polyethylene extruded products have been used for pipe insulation and have also been applied about structural members to provide a resilient outer cushion member. For example, foamed polyethylene cushion members have been applied to supports of gymnastic equipment, playground equipment, football standards and basketball poles to reduce the possibility of injury by striking of the structural member.
Foamed polyethylene can be extruded in a number of different shapes and is very valuable for the type of applications described above. Unfortunately, the product is relatively soft, and thus, the outer surface can tear easily, even though there is a very thin skin portion produced at the outer surface of the product during the extrusion process. To overcome this problem, fabric or tape have been applied about the product, and thus, provides a further surface which protects the underlying polyethylene from damage.
It would be desirable to have a foamed polyethylene product which has a tougher outer surface and one which can be produced in a cost effective manner. Some solutions to the above problem have been proposed and one such solution involves using a separately foamed cylindrical sheath, which when exposed to heat shrinks about a product. The outer sleeves are placed about a polyethylene foamed product and then heat is applied to the sheath which then contracts to the diameter of the foamed polyethylene. This results in a two-stage process to marry the polyethylene foamed body and the outer sheath and it also requires somehow placing the foamed polyethylene body within the outer sheath. Examples of these types of structures and other arrangements are disclosed in U.S. Pat. Nos. 3,607,497, 3,813,372, 3,832,260, 4,634,615, 4,776,803, 4,780,158, 4,861,412, 4,950,352 and 5,360,048.
The present invention seeks to address the problems outlined above and produce a product which can be produced at lower cost.
A polyethylene product according to the present invention comprises a foamed body portion of polyethylene in combination with an outer skin of non-foamed thermal plastic polyethylene which is fused to the foamed body portion.
According to an aspect of the invention, the foamed body portion has a boundary layer which has undergone thermal degration and acts an intermediary securing by heat sealing the outer skin to the body portion.
According to a further aspect of the invention, the skin includes a significant amount of colour pigment and the body portion has no appreciable or consistent colour pigment. portion.
According to a further aspect of the invention, the outer skin includes a significant amount of ultraviolet stabilizers and the body portion does not have any significant amounts of ultraviolet stabilizers.
According to a further aspect of the invention, the polyethylene product with the body portion and the outer skin is of a generally cylindrical shape with an open central cavity which is placed about an elongate member, such that the elongate member is within the cavity whereby the polyethylene product provides an outer resilient sheath about the elongate body member.
A method of manufacturing, according to the present invention, produces a foamed polyethylene product having an outer tough sheath about a foamed polyethylene body portion. The method comprises extruding the foamed polyethylene body portion of a desired cross section, partially cooling the extruded body portion, and subsequently extruding a thin thermal plastic skin directly onto the body portion, resulting in fusing or heat sealing of the skin to an upper layer of the body portion, which due to the heat of the extruded skin, causes limited collapse of a boundary layer of the foamed body portion and securement of the skin to the body portion.
According to an aspect of the invention, the step of extruding the skin is conducted in-line with the step of extruding the polyethylene body portion and prior to complete cooling of the polyethylene body portion.
According to a further aspect of the invention, the method includes a step where the polyethylene body portion is pulled through a cylindrical extruding die which applies the extruded skin to the foamed body portion as it is pulled through the die.
According to a further aspect of the invention. The extruding die applies a skin having a thickness of approximately 0.003 inches.
According to a further aspect of the invention, the foamed body and the skin are of different colours.
According to yet a further aspect of the invention, the cylindrical die includes a control arrangement for adjusting the extrusion discharge rate from the cylindrical die and the method includes adjusting of the control arrangement to produce a desired outer skin thickness.